1. Field of the Invention
The present invention relates to methods for determining a sheet height in a sheet transportation unit, in particular in order to separate sheets that extend beyond a predefined height from other sheets not extending beyond that height. The invention also relates to a printing system that includes a sheet transportation unit and an optical sensor for determining a sheet height.
2. Description of the Related Art
In a known print process, a sheet of a medium, such as paper, is separated from a stack and conveyed by a sheet transportation unit towards a print process unit for having applied marking material, such as ink, on a side of the sheet directed to the print process unit, while the sheet passes the print process unit. The distance between the print process unit and the sheet, also known as the print gap, is usually kept small, in the order of 1 to 2 mm, for achieving a high print quality. Because of this small distance, elements of the print process unit, such as print heads, are easily touched by a sheet that passes these elements, particularly if the sheet may have defects that extend above a regular, nominal height over a supporting surface of the sheet transportation unit. Since these touches may lead to degradation of the achieved print quality, to faults in the print process unit, such as nozzle failure in the case of inkjet printing, or even to sheet jams that block the transportation path for further sheets, it is needed to prevent any contact between the print process unit and the moving sheet.
A possible solution for preventing a contact between a sheet and a print process unit is to determine a sheet height of a sheet before it comes to the print process unit and to eliminate from the transportation unit sheets that have a sheet height above a predetermined height limit. A sheet height may be determined by optical means, such as an optical sensor that applies electromagnetic radiation and an optical triangulation technique to determine a spatial location of sheet surface points from data derived from the projected and reflected radiation. The height of the sheet surface points are collected in a two-dimensional raster image, wherein a pixel is associated with a part of the scanned surface and a pixel value represents a local height of the sheet, as sampled by the optical sensor.
However, a problem exists in deriving a criterion for rejecting a sheet to proceed to the print process unit. A limit may be set for a height value that occurs for any pixel in the height image. If this limit is too high, sheets may pass that give problems with the print process unit. These sheets are called false positives. If a limit is set too low, too many sheets will be discarded, that would not have given any problem if allowed to pass towards a process unit, thereby unnecessarily lowering the productivity of a printing system. These sheets are also known as false negatives. At the same time, the height value of every pixel has to be reliable. A height value of a pixel corresponds to a height of a part of the sheet as measured by an optical sensor using a radiation spot size and a predetermined size of a detection element. It has experimentally been found that a deviating height value arises for a part of the sheet that is not sufficiently flat with respect to the optical sensor. These parts thereby have an uncontrolled influence on the sheet height determination. In particular, deviating height values occur at the sheet edges, where pixels are partly associated with a sheet surface part at the sheet edge and partly with a supporting surface of the sheet transportation unit, since these pixels overlap an edge of the medium sheet.
It is an object of the present invention to handle deviating height values and their influence on the sheet height determination.